[html-formfu] name and id are missing from customized Checkboxgroup elements

[Carl Franks]

It's your lucky day ;)

I've committed an update to support attributes_xml
You can use both attributes + attributes_xml - they'll get merged.

I've also added the shorter versions - attrs, attrs_xml, label_attrs,
label_attrs_xml
You can NOT use both a long and short version of a single option in
the same list item.

Oh, and it error checks too, so it'll die if you use an unknown key.

Carl